1. Field of the Invention
The present invention relates to an optical scanning device used in a tandem system color image forming apparatus such as color copier, color printer and color facsimile apparatus.
2. Description of the Related Art
Recently, the tandem system color image forming apparatus is used in order to upgrade the forming speed of color images substantially. In this color image forming apparatus, four photoreceptor drums are arranged in a row along the conveying direction of the printing paper. The photoreceptor drums are uniformly charged by a charging unit, and the photoreceptors are simultaneously exposed to laser beams, and four electrostatic latent images are formed. The four electrostatic latent images are developed by yellow, magenta, cyan, and black toners in a developing unit, and are sequentially overlaid and transcribed, and a color image is formed. This color image forming apparatus is widely used in digital copiers and laser printers.
Such tandem system color image forming apparatus generally comprises, as shown in FIG. 5, optical scanning devices 6a, 6b, 6c, 6d, photoreceptor drums 2a, 2b, 2c, 2d, and developing units respectively corresponding to yellow, magenta, cyan, and black colors. The optical scanning devices 6a, 6b, 6c, 6d are composed of deflectors 22a, 22b, 22c, 22d, first fxcex8 lenses 24a, 24b, 24c, 24d and second fxcex8 lenses 25a, 25b, 25c, 25d for composing converting optical units, reflection mirrors 23a, 23b, 23c, 23d, and focusing optical units 26a, 26b, 26c, 26d. 
On the other hand, a four-pass system color image forming apparatus transcribes yellow, magenta, cyan, and black colors to intermediate transcribing elements one by one, and transcribes on the printing paper in batch. That is, the four-pass system color image forming apparatus is composed of four developing devices, one optical scanning device, and one photoreceptor drum. Accordingly, the four-pass system color image forming apparatus is smaller in size, but slower in printing speed.
By contrast, the tandem system requires a greater number of optical scanning devices and photoreceptor drums than in the four-pass system, and the entire apparatus is larger in size and higher in cost.
To solve these problems, it has been attempted to integrate the optical scanning device so as to reduce size and cost while maintaining the features of high speed printing of the tandem system.
Above all, it has been proposed to reduce the number of deflectors, that is, polygon motors which are most expensive among components of the optical scanning device, from four pieces to one or two. This is a technology of emitting plural laser beams to one polygon mirror surface. For example, plural laser beams are synthesized by composite members of prism and half-mirror, and emitted to a same position on the polygon mirror, or without being synthesized, parallel or alternate beams are emitted to the polygon mirror surface. Or, to decrease the number of optical lenses, four laser beams are designed to pass the same optical lens.
These technologies contribute to reduction of size and cost of the optical scanning device, but lead to other problems such as ghost printing or color overlaying defect due to defective space separation of laser beams of the optical scanning device.
In the system of synthesizing four laser beams, or arraying in parallel beams to pass through a same lens, if the space separation of laser beams after passing through the lens is defective, for example, if the laser beam for printing yellow mixes into cyan for next printing to print cyan color in blank area or printed area of the printing paper, this problem occurs.
Such color overlaying defect, in particular, ghost printing is a serious problem in the case of a printer in which more accurate and clean printing is required than in a copier.
It is hence an object of the invention to present an integrated optical scanning device capable of preventing color overlaying defect of image formed by plural emitted laser beams.
An optical scanning device of the invention comprises:
a deflector for deflecting in batch a plurality of laser beams entered as being arrayed in a vertical direction to the plane of deflection;
a converting optical unit for converting equiangular speed motion of the plurality of laser beams deflected by the deflector in batch into uniform speed linear motion;
a plurality of reflection mirrors for reflecting the plurality of laser beams converted by the converting optical unit;
a plurality of focusing optical units for focusing the plural laser beams reflected by the plurality of reflection mirrors; and
a plurality of photoreceptors for forming an electrostatic latent image by the plurality of laser beams focused by the plurality of focusing optical units, each of the plurality of photoreceptors being arranged in a sub scanning direction to each of the plurality of laser beams from the plurality of focusing optical units,
wherein a yellow image is formed by a laser beam reflected by a remotest reflection mirror from said deflector, among said plurality of reflection mirrors.
In this configuration, if light of defective space separation component of other laser beam mixes into the laser beam for forming the yellow image, since yellow is the least obvious color, color overlaying defect of image can be prevented, and an image of high quality is obtained.